Since there can't be congestion and locks with boats I don't see why we can't connect with canals owned by other players. (like we can with roads)
So I'd suggest to allow connecting to any canal !
How does this work in the real world? I would think than canals built by improving existing rivers would be allow this, while fully artificial canals are more private. But there aren't many canals in my part of the world.
Well, in real life there's congestion.
To the matter : the companies having the canals live from the tolls they collect. If someone builds canals adding potentially more destinations (and thus traffic and thus more income) they sure want them to be connected to their own network.
But really what matters here is gameplay and the current way is both not coherent with how roads work and a pain in multiplayer (the new map from a128.entropy is rather 'canal intensive' and we want/need others to be able to connect to our canals and still retain ownership to save on tolls - or for security)
I don't think there are any private shipable canals anywhere in the world to begin with.
There are, but they are smaller and don't have big economical influence. They can't be compared with Panama or Suez Canals.
It's hard to find any info about them, for the obvious reason they don't need to give any. You have the canal and if someone near thinks it's nice to use they go talk with you.
But can you just start digging into the Suez and Panama Canals? The latter probably wouldn't like others starting to drain Gatun Lake.
There are some important canals, like the bridgewater canal: http://en.wikipedia.org/wiki/Bridgewater_Canal (http://en.wikipedia.org/wiki/Bridgewater_Canal) The first artifical canal in England and never privatized. Many more were built privately, but got nationalized later on.
EDIT: incorporated in r5929
Thanks Prissi ! :)